Method for producing shaped articles from polymers and copolymers of acrylonitrile



States 3,073,669 METHOD FOR PRODUCING SHAPED ARTICLES FROM POLYMERS ANDCOPOLYMERS F AC- RYLONITRILE This invention relates to a method ofproducing filaments or fibers from polymers and copolymers ofacrylonitrile. More particularly, this invention relates to an improvedmethod of coagulating acrylonitrile polymer from its solution in 69% ormore nitric acid.

Various methods of producing shaped articles and especially fibers frompolyacrylonitrile and from copolymers or interpolymers of a majorproportion of acrylomtrile and a minor proportion of another monomer ormonomers heretofore have been suggested. Among these methods, a reviewwill be given with respect to several methods for the preparation offibers wherein polyacrylomtrile be dissolved in an inorganic solvent andthe resulting solution be subjected to wet spinning.

In U.S. Patent No. 2,140,921 a concentrated aqueous solution containingsuch inorganic metal salts as. zinc chloride, sodium thiocyanate,calcium thiocyanate, lithium bromide and aluminium perchlorate areproposed as solvents for polyacrylonitrile. Rein: Agnew. Chem. 60, 159(1948), Keln: J. Prakt. and Kanbara: Japanese Patent No. 147,451,disclose that an inorganic acid solution is usable as a solvent forpolyacrylonitrile. As pointed out by researchers of E. I. du Pont deNemours & Co., however, these inorganic solvents commonly have seriousdrawbacks and therefore they arenot so suitable for the production ofshaped articles from polyacrylonitrile. Recently these inorganicsolvents have been improved to some extent, for instance, in CresswellU.S. Patent No. 2,558,730 which relates to thiocyanate-containingsolvents, Stanton U.S. Patent Nos. 2,648,592-3 which relate tochloride-containing solvents, and Fujisaki et a1. Japanese patentpublication Nos. 984- 5/1957, 1,443/1957, 9,516/1957, 10,866/1957 and965/ 1958 which relate to nitric acid-containing solvents. Thus theproduction of polyacrylonitrile fibers wherein aqueous solutions ofinorganic acids and the salts thereof are used as a solvent forpolyacrylonitrile is now rapidly progressing.

In the case of using nitric acid as a solvent forpolyacrylonitrile, itshould be noted also that all of the defects accompanying the use ofaforesaid inorganic solvents are not completely removed. One such defectis the high viscosity of the spinning solution. However, the viscosityis far lower than that with other inorganic compounds, such as ordinaryinorganic salt concentrated aqueous solution solvent or phosphoric acidcontaining spinning solution, but is highly viscous compared withorganic solvent spinning solutions.

Even when nitric acid is employed as the solvent there is somedifficulty in the treatment of the solution due to its relatively highviscosity. It is difficult to maintain the spinning solution at hightemperatures or to heat the coagulating bath to lower the viscosity ofthe spinning solution during extrusion since such heating tends tohydrolyze the nitrile group in the polyacrylonitrile molecular chain. Asthe results of our extensive studies on spinning processes employinginorganic and organic solvents, it has now been found that the less adifierence between the temperatures of a spinning solution and acoagulating bat the better $59 h PQ of t n Chem. 160, 281 (1942),-

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parency of coagulated filaments and transparency as well as luster offibers after stretching is obtained. Results serving these purposes wereobtained when a difference between the temperatures of the aforesaidspinning solution and coagulation bath was less than 5 C.

In comparison with a dry spinning process or the extrusion of a spinningsolution containing an organic compound e.g. dimethyl formamide as asolvent into a coagulating bath containing glycerine, another defectaccompanying the use of a concentrated solution of nitric acid is thatduring a period of coagulation it is hardly possible to subject to atension the extruded filaments in a fluid state. As observed in a dryspinning process, if it is possible to subject to a tension the extrudedfilaments in a fluid state, it will be of great advantage because thatdesired fineness of fibers can be obtained by applying a suitable degreeof tension and that the nozzle having holes of relatively greaterdiameter can be used.

As will be known by those skilled in the art, the use of a nozzle havingholes of a greater diameter is advantageous in the operation ofextrusion. For instance, when the aforesaid'organic compound is used asa solvent for polyacrylonitrile, the nozzle having holes of diameter of0.22 mm. can be used inorder to obtain fibers having fineness of threedeniers. On the other hand, as seen in Cresswell U.S. Patent Nos.2,648,592-3 which deal with an inorganic solvent containingthiocyanates, the nozzle having holes of a diameter of about 0.05 mm.should be employed in the extrusion of a spinning solution consisting ofthe inorganic -solvent.j Such disadvantage originally results from thefact that there is not any possibility to provide a tension toextruded'filaments in a fluid state.

It is necessary to maintain a nitric acid concentration of from 47% to50% in the coagulation bath to elongate the coagulating thread-likematerial in the coagula= tion process in a fluidizing manner inorder toimprove various properties of the fiber, which is one of the objects ofthe present invention.

While the present invention will, for convenience, be described inconnection with fiber, shaped articles produced by thepresent inventionmay include filaments (monoand multifilaments), foils, ribbons, films,sheets, etc. Since many diiferent embodiments of the invention may bemade by those skilled in the art without departing from the spirit andscope thereof, it is to be understood that we do not limit ourselvesexcept as set forth in the description and claims which follow.

An object of the present invention is to provide an improved method forcoagulation of solutions of acrylonitrile polymerization products, saidmethod suiting for the production of fibers having good mechanicalproperties, e.g. toughness, resilience, and excellent loop elongation inaddition to fine appearance. i

Another object of this invention is to provide a method of producingnon-brittle, tough shaped articles from acrylonitrile polymerizationproducts.

Still another object of the present invention is to provide theproduction of fibers having excellent loop elongation. T

Further object of the present invention is 'to provide the production ofpolyacrylic synthetic fibers having fine lustre and good resilience.

Other objects, features, capabilities and advantages which arecomprehended by the invention will be appar: ent from the descriptionand claims which follow.

According to the present invention, an acrylonitrile polymerizationproduct is dissolved in 69% -or more nitric acid, the resulting solutionis extruded. into the first liquid coagulating bath having approximatelysame temperature as that of said solution and having 47% to 50 .Ilitrig,acid, h Q1 dedv mes is t etchedto desired times its original length in afluid state while passing through said bath, then the extruded andstretched mass is passed through the second liquid coagulating bathcomprising mainly water thereby to form a shaped tough gel of saidpolymerization product which can be molecularly oriented by wetstretching, and the thus shaped gel is dried.

Acrylonitrile polymerization products used herein as a starting materialinclude polymers and copolymers comprising about 85% or more thanacrylonitrile and the mixtures thereof. These copolymers includeacrylonitrile copolymerization products of acrylonitrile with a monomerbeing copolymerizable therewith. Illustrative examples of monomers whichmay be copolymerized with acrylonitrile are compounds containing asingle CH =C group, for instance the vinyl esters and especially thevinyl esters of saturated aliphatic monocarboxylic acid, e.g., vinylacetate, vinyl propionate, vinyl butyrate, etc.; acrylic and alkacrylicacids (e.g., methacrylic, ethacrylic, etc.) and esters and amides ofsuch acids (e.g., methyl, ethyl, propyl, butyl, etc., acrylates andmethacrylates, acrylamide, methacrylamide, N-methyl, -ethyl, -propyl,-butyl, etc., acrylamides and methacrylamides, etc.); similar compoundsof saturated aliphatic dicarboxylic acids (e.g. itaconic, maleic, etc.)and the anhydrides of such acids; methacrylonitrile, ethacrylonitrileand other hydrocarbon-substituted acrylonitriles; numerous other organiccompounds which are copolymerizable with acrylonitrile. A suitablemethod of polymerizing the monomeric acrylonitrile or mixture ofmonomers is a redox polymerization or a'sus'pension polymerization in aheterogeneous phase or an emulsion polymerization using a dispersingagent. Other polymerization methods, however, also may be employed. Theacrylonitrile polymerization product may be of any suitable weightaverage molecular weight, and ordinarily will be within the range of30,000 to 150,000 or higher.

In the practice of this invention into effect the acrylonitrilepolymerization product is then dissolved in a nitric acid from which theacrylonitrile polymerization product is slowly coagulated when thesolution is brought into contact (e.g. immediately after extrusion) witha coagulating solution fully described hereinafter.

It will be understood by those skilled in the art that the solution ofthe acrylonitrile polymerization product in nitric acid should be ofsuch a concentration that a workable viscosity is obtained. Theconcentration of the acrylonitrile polymerization product will depend,for example, upon the solvent composition and extrusion apparatusemployed, the type of shaped article to be formed and the weight averagemolecular weight of the acryacrylonitrile polymerization product, whichusually is within the range of 30,000 to 150,000. The concentration ofacrylonitrile polymers in spinning solution may range, for example, from5% to 20% and/or more by weight. However, improved fibers are obtainedbyemploying a spinning solution having a viscosity of at least 1000poises. Also it is noted that this solution when extruded into acoagulating bath should be maintained at the temperature of from C. to10 C.'or lower.

In the present invention the solution of the acrylonitrilepolymerization product is extruded into a coagulating bath and thenstretched in a fluid state while passing through said bath. Thecoagulating bath used herein should have a concentration of 47% to 50%nitric acid. Highly concentrated nitric acid, however, has notpreviously been employed for the coagulationbath in known spinningprocesses. An industrial advantage, of highly concentrated nitric acidas the coagulation bath is the reduced expense of recovery andpurification for cyclic use. The mass which'has beenstretched inv afluid state i then passed through the second liquid coagulating'bath inwhich said mass is completely-coagulated. It is not so preferred to havemuch time for transferring said mass from the first coagulating bath tothe second one.

With respect to nitric acid used as solvent, the solvent power forpolyacrylonitrile is as follows:

solved. Polyacrylonitrile is well swollen 5 C. Polyacrylonitrile isdissolved Considerably low temperature.

52.47 54% 6: higher The above data show that the critical concentrationof an aqueous nitric acid solution for polyacrylonitrile isapproximately 53% In order for stretching advantageously the extrudedmass five times or more in a fluid state in the first liquid coagulatingbath, however, it is preferred to have the nitric acid concentration ofthe coagulating bath 47% or higher. Thus, the extruded mass may bestretched at a desired ratio in the first liquid coagulating bath havingthe suitable concentration of nitric acid.

Then the stretched mass is passed into the second liquid coagulatingbath containing less than 35% of nitric acid or nitric acid salt or themixture thereof, and it is completely coagulated in said bath and thenwashed with water.

It is necessary that the concentration of the second liq'uid coagulatingbath is lower than that of the first liquid coagulating bath.

To coagulate the fiber conveniently in the gelled state, the temperatureof the second coagulating bath should be as low as possible, but withinthe range of from +40 to 5 C. The temperature of the second coagulatingbath, however, does not affect the properties of the finished product.As will be understood from the above description, the thread in thefirst liquid coagulating bath consisting of an aqueous solution having arelatively higher concentration of nitric acid is not in perfectlycoagulated form and therefore it can be stretched by a little force likea fluid. That is to say, it is treated under such a state that bindingforce between molecular chains of polymer is remarkably weakened. Thisforce may cause shear within the fluid, and very desirable molecularorientation can be promoted at a low temperature. In forming a perfectgel structure in the second coagulating bath, the characteristicmentioned above may promote formation of a gel structure which isdesirable for subsequent heat stretching. Furthermore when a stretch,that is, shear applied to the fluid in the first coagulating bath, isvery high, the swollen mass in which molecular chains are oriented tendsto be converted into more compact form. Such tendency is moreeffectively developed when the higher concentration of polymer isemployed. By our discovery, therefore, if a very high degree of shear isapplied in the first coagulating bath, it is possible to transfer theextruded and stretched mass directly from the first coagulating bath toa water-washing bath without passing through the second coagulatingbath.

For the gelled thread, passed through the first coagulating bath andsecond coagulating bath and the washing "bath, the heat stretchabilityis affected by the stretching ratio of the tension in a fluid state inthe first coagulating bath. That is to say, the more increasing thestretch ratio in the first coagulating bath, the more decreasing theheat stretching ratio.

Moreover, a factor of shear of the spinning solution on the nozzle isaccompanied with this relation, its efiect, however is evident by thetable followed which is a part of our experimental results.

TABLE I.-THE RELATION BETWEEN THE STRETCHING RATIO IN THE FIRSTOOAGULATING BATH AND THE HEAT STRETCH RATIO Stretching ratio in the 1stcoag. bath (times) Rate of shear bath 2 5 l 15 20 25 30 50 m./ min.) 1

Heat stretching ratio 1 Rate of shear is indicated in linear velocity ofspinning solution at the time of leaving the nozzle.

As mentioned above, the application of stretching to an extruded masshaving fluid properties in the first coagulating bath is highly suitablefor orientation of polymer molecules, and the behaviors of these polymerchains are similar to those in melt spinning. In a conventional methodwhich comprises extruding a solution of the polyacrylonitrile in aconcentrated aqueous solution of nitric acid into a coagulating bath andcoagulating the extruded mass while passing through said bath, the fluidis molec ularly oriented to some extent due to nozzle shear, but ittends to shrink immediately after the extrusion from the nozzle. In thiscase, therefore, it is impossible to stretch the extruded mass one ormore times to its original length. Such shrinkage which usualy extendsover 10 to 70% or higher and which happens in a coagulating bath hasentanglement of molecular chains promoted. The coagulated filament whichhas a coagulated gel structure produced under the above said situationswill be harmed by great constrained forces against individual molecularchains or micell groups when said filament is subject to heat stretchingin the subsequent process. As its result, fibers obtained would bebrittle. Consequently speaking, there is a remarkable difference betweena polyacrylic fiber of the present invention and that of a conventionalmethod in the point of brittleness. In the present invention,brittleness will be expressed in ball mill value. When a certain amountof fibers are ground in a particular ball mill for 2 hrs. and then thefinely powdered fibers are weighed, the ball mill value is calculatedaccording to the following manner:

=ball mill value (percent) Now acrylonitrile polymer is dissolved in anaqueous 70% nitric acid solution to prepare a spinning solution. Inaccordance with a conventional method, the spinning solution is extrudedinto an aqueous 30% nitric acid solution to prepare coagulatedfilaments. After" washing with water, the filament is heat-stretched toseven times its original length. The thus produced mass has a ball millvalue of 67%. According to the present invention, the above spinningsolution is extruded into the first coagulating bath being an aqueous49% nitric acid solution and the extruded mass is stretched thirteentimes its original length.

After coagulating in the second coagulating bath in an aqueous 28%nitric acid solution, the extruded and stretched mass is furtherheat-stretched seven times its original length. The thus produced masshas a ball mill value of 5% In a conventional process the above spunfiber is unable to be heat-stretched more than eight times its originallength. In the present invention, however, it is noted that the spunfiber can be heat-stretched up to fourteen times its original length,and higher order of stretching, the treated fiber can obtain severalfeatures of desirable mechanical properties such as lustre of the fiber,strength, etc. Further-more loop elongation of the fiber may be improvedcorresponding to the decrease of brittleness.

As stated hereinabove, one of the characteristics of the inventionresides in the fact that a highly viscous spinning solution can bereadily worked. Such a fact will permit to have the more higherconcentration of polyacrylonitrile in a spinning solution, whereby theproductivity of fiber per unit volume of solvent is developed. At thesame time the thus produced fiber may be provided with good propertieswhich are beyond our expectation. That is to say, the fibrillation ofthe fiber is scarcely observed and the resilience of the fiber isremarkably improved.

Heretofore it was found that there was a limitation in the molecularweight of polyacrylonitrile employed when nitric acid was used as thesolvent. In accordance with the present invention, however, anacrylonitrile polymerization product having a molecular weight higherthan that of any acrylonitrile polymer heretofore used may be used forpreparing a spinning solution having an ordinary concentration of saidpolymerization product. In the present invention, for instance,polyacrylonitrile having a molecular weight of 100,000 or more than canbe used. This fact also serves to improve the strength of fiber.

Furthermore, the fiber produced by the present invention can be easilyrelaxed by heattreatment after drying said fiber. For example,polyacrylonitrile fiber can be relaxed by dry heat treatment and show aloop elongation of over ten percent. By wet heat treatment, this fibercan be relaxed at the lower temperature, e.g. C., than the relaxingtemperature of any conventional po-lyacrylic fiber.

The coagulating bath used in the present invention should be at atemperature of less than 20 C. and preferably less than 10 C. But it isunobjectionable that a water-washing bath is warm water having atemperature of less than 70 C.

Additionally speaking, it is not only unobjectionable to carry outstretching at a low ratio in a second coagulat ing bath, but it israther preferred. If necessary, however, also it is allowed to shrink tosome extent in the said bath, as occasionally observed in a conventionalspinning process.

The following examples, will serve to illustrate this invention further,without however limiting the same. All molecular weights ofacrylonitrile polymerization products indicated herein are calculatedfrom the intrinsic viscosity of dilute solutions of said products indimethyl 'formamide. In order to attain the object of the presentinvention to improve the fiber as indicated previously, it isindispensable that nitric acid having a concentre-tion above 69% beemployed as solvent to prepare spinning solution having a viscosity ofmore than 1,000 poises and the spinning solution be extruded into acoagulation bath having a nitric acid concentration of from 47% to 50%.It is impossible to separate the use of highly concentrated nitric acidas solvent, the use of a highly viscous spinning solution and the use offrom 47 to 50% nitric acid for the coagulation bath. In the spinningprocess, wherein the spinning solution is comprised of nitric acid assolvent, in which spinning solution is extruded into diluted aqueousnitric acid solution, the concentration zone (of the coagulation bath)for the nitric acid which enables spinning decreases with an increase inthe concentration of solvent, since the concentration of raw nitric acidin the coagulation bath varies inversely with the concentration ofnitric acid in the spinning solution. The primary factor which dominatesthe phenomenon of physical variation in the coagulation step isdiffusion of solvent in the coagulation bath. The same effect asobtained by this invention cannot be obtained by materially lowering thenitric acid concentration in the spinning solution and increasing thenitric acid solution concentration in the coagulation bath since 7 theviscosity of the spinning solution will be too low. Only when the statedrelationship between the concentration of nitric acid as solvent in thespinning solution, the concentration of nitric acid in the coagulationbath and the viscosity of the spinning solution is satisfied, will theadvantage of the instant invention result.

Example 1 Polyacrylonitrile having a molecular weight of 92,000 wasdissolved into 70% nitric acid aqueous solution at 3 C. for 3 hours tohave a concentration of 18%. The resulting spinning solution wasextruded into 48% nitric acid aqueous solution at C., and the extrudedfilament was stretched to 24 times its original length whilecoagulation. Immediately thereafter, the filament was fed to an aqueouscoagulation bath and further stretched twice. The filament was passedinto a water-washing bath and washed with water. Then the washedfilament was subjected to heat stretching to 5 times its original lengthin superheated steam (270 C.) The produced fiber had a strength of 5.8gr./d., a ball mill value of 3% and a loop elongation of 7.3%.

For comparison, the spinning solution used in the above example was spunin accordance with a previously known method. That is to say, thespinning solution was extruded into 34% nitric acid aqueous solution.The extruded filament was stretched twice its original length in thecoagulating solution. After washing with water, the coagulated filamentwas subjected to heat stretching to 5 times its original length therebyto obtain fiber having a strength of 3.8 gr./d., a ball mill value of54% and a loop elongation of 2.8%.

Example 2 Polyacrylonitrile having a molecular weight of 74,000 wasdissolved into 70% nitric acid aqueous solution at C. to have aconcentration of 18%. The spinning solution resulted was extruded into50% nitric acid aqueous solution at -7 C., and the extruded filament wasstretched to 90 times its original length while coagulation. Thereafter,the filament was passed into 25% nitric acid aqueous solution to form acompletely coagulated filament and then it was washed with water. Thenthe washed filament was stretched to 3 times its original length byheating in hot water at 100 C. The fiber thus produced had a strength of4.2 gr./d., a ball mill value of 5%.

In accordance with a previously known method, the spinning solution usedin the above example was extruded into 30% nitric acid aqueous solutionand it was coagulated while shrinking to the order of 5%. After heatstretching, the produced fiber had a strength of 3.7 gr./d., a ball millvalue of 50% and a loop elongation of 2.4%.

Furthermore, the spinning solution could be spun into film by extrudingit in the coagulating bath consisting of 49% nitric acid aqueoussolution cooled to 0 C. and in the coagulating process the film ingelled state was stretched times to improve the properties of thefinished product. The film which was in the form of gel was very thincomparing that produced by published method, and clear, and had a markedtoughness and resilience.

Example 3 Polyacrylonitrile having molecular weight of 74,000 wasdissolved into 70% nitric acid aqueous solution at 5 C. to have apolymer concentration of 30 gr. per 100 ml. and thus the spinningsolution was prepared. The nitric acid used herein was purified prior toits use to contain 0'.0002% of nitrous acid. The spinning solutionprepared above showed a viscosity of 8,000 poiscs at 0 C.

The spinning solution was extruded into 48% nitric acid aqueous solutionat 0 C., and the extruded filament was stretched 24 times its originallength in the coagulating solution. The filament was fed to an aqueouscoagulation bath wherein it was further stretched three times, waspassed into water-washing bath and it was washed with water until it wasfree from nitric acid. Then the filament Was stretched to 5 times itsoriginal length in 300 C.--superheated steam thereby to obtain fiberhaving a strength of 5.2 gr./d., a ball mill value of 1%, and a loopelongation of 7.4%.

In the above experiment, the spinning conditions were same as in Example1, excepting that the spinning solution having a very high concentrationof acrylonitrile polymer was used. The result of this example, however,showed an increased strength and an improved ball mill value, ascompared with those of Example 1.

Example 4 Polyacrylonitrile having a molecular weight of 120,000 wasdissolved in nitric acid aqueous solution to have a polymerconcentration of 8 gr./ 100 ml. and a solution having a viscosity of5,000 poises was obtained. The used nitric acid was purified prior toits use to contain 0.0004% of nitrous acid.

This spinning solution was extruded into 48% nitric acid aqueoussolution at S C. and the extruded filament was stretched in a fluidstate to 47 times its original length while coagulation. The filamentwas passed to an aqueous coagulation bath wherein it was furtherstretched 2 /2 times. The filament was passed into waterwashing bathwherein it was washed with water, and then it was stretched to 5 timesits original length in 300 C.- superheated steam. Theproduced fiber hada strength of 6.5 gr./d., a ball mill value of 5% and a loop elonga tionof 7.8%.

Example 5 A fiber produced by following the procedures of Example 3 wassubjected to relaxing treatment in steamy atmosphere having a relativehumidity of 40% at 100 C. for 10 seconds, and the loop elongation of thefiber thus treated was raised to 17.3%.

On the other hand, such a fiber as prepared by any conventional method,for instance a method indicated in the last paragraph of Example 1, wassubjected to relaxing under the same conditions as in above. As itsresult, the loop elongation of the treated fiber was depressed to 1.8%and there was observed adverse effect in such treatment.

Example 6 A copolymer of acrylonitrile containing parts of acrylamideand having an average molecular weight of 74,000 was dissolved in 67%nitric acid aqueous solution at 0 C. to have a concentration of 11%. Theresulted solution was extruded into 47% nitric acid aqueous solution at0 C. and the extruding filament was stretched ten times original lengthwhile coagulation, and then transferred to the second liquid coagulatingbath consisting of the aqueous solution of 30% nitric acid at 5 C. toform a perfectly coagulated filament having gel structure, and thenwashed with water, and stretched 7 times by super heated steam. Theproduced fiber had a strength of 3.2 g./d., a ball mill value of 8% anda loop elongation of 12.0%.

When a spinning solution is prepared by dissolving polyacrylonitrile orcopolymer containing at least by weight of acrylonitrile it is desirablethat a solvent therefore has high solvent power for polymer and thatdissolution condition of the polymer molecule in the resultant solutionis very pertinent. To obtain such a spinning solution, the solvent mustbe one which can dissolve the polymer in higher concentration withoutgelling the resultant solution at lower temperature, maintaining goodspinability of the solution. Such a solvent is nitric acid of 69% ormore. In case the solvent is 69% or more nitric acid, the nitric acidconcentration of the first coagulation bath must be 4750%. In order toimprove mechanical properties of the product fiber such as loopelongation and ball mill value, the spinning solution must haveviscosity more than 1000 poises.

What we claim are:

1. A method of producing shaped article from acrylonitrilepolymerization product comprising at least 85% by weight ofacrylonitrile, which comprises dissolving said polymerization product innitric acid having a concentration of at least 69% so that the resultantsolution has viscosity of more than 1000 poises, extruding the solutioninto a coagulation bath containing 47-50% nitric acid, stretching theextruded mass in the coagulation bath, washing the stretched mass withwater, heat-stretching the washed mass until the molecules areorientated and then drying.

2. In the coagulation from a spinning solution and in a coagulation bathof acrylonitrile polymerization product containing at least 85% byWeight of acrylonitrile units, the improvement wherein (a) the viscosityof the spinning solution is more than 1000 poises, (b) the spinning 10solution consists essentially of the arcylonitrile polymerizationproduct and aqueous nitric acid solution, the concentration of nitricacid in the aqueous nitric acid being at least 69%, and (c) thecoagulation bath consists essentially of aqueous nitric acid, the nitricacid concentration of said coagulation bath being from 47 to 50%.

References (Iited in the file of this patent UNITED STATES PATENTS2,140,921 Rein Dec. 20, 1938 2,579,451 Polson Dec, 18, 1951 2,670,268Stanton Feb. 23, 1954 2,716,586 Terpay Aug. 30, 1955 2,790,700 StantonApr. 30, 1957 2,878,097 Halbig Mar. 17, 1959 FOREIGN PATENTS 721,947Great Britain Jan. 19, 1955

1. A METHOD OF PRODUCING SHAPED ARTICLE FROM ACRYLONITRILEPOLYMERIZATION PRODUCT COMPRISING AT LEAST 85% BY WEIGHT OFACRYLONITRILE, WHICH COMPRISES DISSOLVING SAID POLYMERIZATION PRODUCT INNITRIC ACID HAVING A CONCENTRATION OF AT LEAST 69% SO THAT THE RESULTANTSOLUTION HAS VISCOSITY OF MORE THAN 1000 POISES, EXTRUDING THE SOLUTIONINTO A COAGULATION BATH CONTAINING 47-50% NITRIC ACID, STRETCHING THEEXTRUDED MASS IN THE COAGULATION BATH, WASHING THE STRETCHED MASS WITHWATER, HEAT-STRETCHING THE WASHED MASS UNTIL THE MOLECULES AREORIENTATED AND THEN DRYING.